In Situ Confocal Laser Scanning Microscopy of AA 2024-T3 Corrosion Metrology

Abstract

The morphology of attack at and around the intermetallic compounds ͑IMC͒ present on bare AA 2024-T3 was studied in situ using confocal laser scanning microscopy. Exposures were conducted in 0.1 M Na 2 SO 4 ϩ 0.005 M NaCl at pH 3, 6, and 10 as well as near-neutral 0.5 M NaCl. The types of attack observed could be categorized as matrix and IMC pitting, trenching adjacent to IMC, and matrix etching. The electrochemical behavior of bulk synthesized Al-Cu, Al-Cu-Mg, and Al-Cu-Fe-Mn intermetallic compounds as well as that of AA 2024-T3 was used to rationalize the observed attack metrology. The galvanic coupling between the AA2024-T3 matrix and the intermetallic particles controlled the attack rates. In Al-Cu-Mg, the strong polarization to the opencircuit potential of the alloy caused rapid dissolution ͑ca. 10 mA/cm 2 ͒, whereas for the Al-Cu-Fe-Mn the dissolution rates were on the order of 100 A/cm 2 . The limited dissolution rates of the Al-Cu-Fe-Mn phase were due to the cathodic polarization of these particles by the matrix under open-circuit conditions. Several pits were initiated at large Al-Cu-Mg particles. These pits were stable within the Al-Cu-Mg phase, but could not form stable pits in the alloy matrix during open-circuit corrosion. Calculation of growth rates and pit stability products for the individual IMC emphasized the role of metastable pitting in the observed corrosion metrology, which developed on AA2024-T3 during open-circuit corrosion.

Electrochemical Characterization of Intermetallic Phases Common to Aluminum Alloys as a Function of Solution Temperature

“…In cases where the bulk alloy, AA7075-T651, was tested, the electrode area was 1cm 2 and testing was carried out using a conventional 'flat-cell' (PAR).…”

“…However, polarization testing was able to reveal significant quantitative insights allowing a leap in understanding the origins of localized corrosion (and in some cases the absence of localized corrosion). Examples include; spontaneous passivity of Mg 2 Si at alkaline pH and the increase in the ability of Fe-containing intermetallics to sustain ORR as either pH or [Cl − ] increase, to name but a couple of important factors. 7,9 Similarly, such richness is not predicted by methods such as SKPFM, which has been previously used to predict localized corrosion on the basis of Volta potentials measured in air; [14][15][16] highlighting that kinetic information, in the electrolyte and conditions of interest cannot be simply substituted.…”

A Summary of Corrosion Properties of Al-Rich Solid Solution and Secondary Phase Particles in Al Alloys

“…Particles consisting primarily of noble elements generally possess a high corrosion potential, and act as a cathode to support the oxygen reduction reaction or hydrogen evolution. The adjacent matrix then dissolves at the matrix/particle interface, resulting in trenching morphology surrounding the particle as shown in Figure 2 [28][29][30][31][32]. However, the galvanic relations between the matrix and secondary phase particles may change with time and as environmental conditions are altered.…”

New insights into the cathodic dissolution of aluminium using electrochemical methods

“…Many experiments have also been performed on aluminium alloys (for instance AA 2024-T3 alloy) in which galvanic corrosion occurs between an aluminium matrix acting as anode and copper rich intermetallic particles acting as cathodes [7][8][9][10]. As a consequence, generation of high pH zones at the surface of the particles promotes chemical dissolution of the Al matrix in the vicinity of the particles.…”